It has been defined in 3GPP TS 23.216 how to support single radio voice call continuity (SRVCC) while a call/session is in a steady state, i.e. the user being active in an IMS session using PS (Packet Switching) media flow(s) at the time of initiation of Access Transfer to CS (Circuit Switching).
FIG. 1 show, as an example, a SRVCC network architecture provided in 3GPP TS 23.216. As shown in FIG. 1, the UE accesses the IMS via E-UTRAN, S-GW/PDN GW. The E-UTRAN, also known as LTE (Long Term Evolution), comprises a number of E-NodeBs responsible for the radio access network. The EPS is simplified as two type of network element: eNodeB and EPC, by merging functionally the NodeBs, RNCs and CNs of the existing WCDMA and TD-SCDMA systems.
The so-called EPS includes EPC (Evolved Packet Core, i.e. the core network), E-UTRAN (also known as LTE) and UE. The EPC comprises: MME (Mobility Management Entity) for acting as controlling node and responsible for signaling processing of the core network; S-GW (Serving GateWay)/PDN-GW (Packet Data Network GateWay) responsible for data processing of the core network, wherein a non 3GPP radio access network may access the EPC via the PDN-GW and a 3GPP radio access network may access EPC via the S-GW.
In addition, FIG. 1 also shows the interface between network elements proposed by this specification. For example, E-UTRANs and EPCs are mutually connected via S1 (similar to lu) interfaces, E-UTRANs are mutually connected via X2 (similar to lur) interfaces (non shown), and UEs and E-UTRANs are mutually connected via LTE-Uu interfaces.
In an environment as shown in FIG. 1, a decision as to handing over an UE to the circuit switching domain provided by UTRAN (UMTS Terrestrial Radio Access Network)/GERAN (GSM/EDGE Radio Access Network) could be made, when the UE is at the edge of the coverage of the E-UTRAN or outside of the coverage. In the UTRAN/GERAN, the UE accesses to the IMS network via a base station, a MSC (Mobile Switch Center) server.
The UTRAN is a rather new access network for UMTS and becomes currently an important access way of the UMTS, and may include NodeB, RNC (Radio Network Controller), CN (Core Network) etc.; the GERAN is a key portion of the GSM drawn up and maintained by the 3GPP and is also included in the UMTS/GSM network, and includes a base station BS and a Base Station Controller and their interfaces (e.g. Ater interface, Abis interface, A interface etc.). In general, the mobile operator’ network is composed of a number of GERANs, which are combined with the UTRAN in the UMTS/GSM networks.
More details about the other network elements in FIG. 1 and the communicating manner therebetween may refer to TS 23.216.
FIG. 2 shows, as an example, the flow of concerned calls from the E-UTRAN to a target GERAN. In order to complete handover of a voice session, the voice call should be anchored in advance for example at the SCC AS (Service Centralization and Continuity Application Server) in the IMS.
As shown in FIG. 2, the source E-UTRAN will send a handover request to a local MME when it decides to hand over an ongoing VoIP call of a local UE from the PS domain to the CS domain based on measurement report received from the local/source UE. Then, the source MME will split the bearer (for subsequent handover of voice service) and send a corresponding handover request from the PS domain to the CS domain to a MSC server or media gateway currently capable of covering the local UE. The corresponding MSC/media gateway will initiate a session handover after having prepared for handover and established a circuit. Herein, it should be noted that the steps 6-9 of the dashed portion (and also steps 20-21) may be omitted in case that the target MSC to which the local UE is to handover is the same one as the MSC receiving the handover request from the MME.
In the following steps (i.e. after step 10), FIG. 2 shows two handover procedures of user layer:                A session handover procedure of IMS layer (at a remote UE, i.e. the other party setting up a VoIP session with a local UE) (steps 11-12). More particularly, the session handover procedure is performed by the SCC AS in the IMS, and then the remote UE is updated by the SDP (Session Description Protocol) of the target CS access leg, and the source EPC PS access leg is released. The above steps would cause the voice portion of the ongoing session to be handed over from the EPC to the MGW at the user layer.        A handover procedure to a target cell at layer 2 (handover from the E-UTRAN to the GSM performed at the local UE and the access network, steps 15-21), which is a handover between RATs performed at the local UE and the access network.        
More details about the steps of FIG. 2 may refer to 3GPP TS 23.216.
However, it is not indicated in the 3GPP TS 23.216 how to support SRVCC during a non steady state of a session. The non steady state may mean the earlier state of a session, for example the calling party receiving a provisional SIP response message (except a 100 Trying message) but not a 200 OK message.
Therefore, a mechanism capable of supporting the SRVCC handover from the PS domain to the CS domain during a non steady state of a session is needed.